Polyimides derived from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane are useful, inter alia, for the production of electronic coatings on semiconductor wafers, such as polysilicon wafers. One way of forming such coatings on the wafers is to apply a solution of a polyamic acid polymer derived from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane to the wafer and thereafter bake the coated wafer to cure (imidize) the resin so that the corresponding polyimide is formed. In order to apply solutions of such polyamic acids to the wafers spin coating procedures are used, and in these operations purity, integrity, and uniformity of the resultant coating are essential.
Unfortunately it has been discovered that in humid atmospheres (e.g., about 55% relative humidity or above) solutions of such polyamic acid polymers in some commonly used solvents (e.g., N-methylpyrrolidone) tend to be unstable in the sense that during spin coating operations undesirable precipitate formation occurs on the rotating surface of the wafer. The result is that the coating loses adhesion to the wafer and flies off of it during spin coating.
The need thus exists for solutions of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane polyamic acid polymers which do not undergo undesirable precipitate formation on rotating wafer surfaces when used in spin coating operations even if conducted under conditions of 55% relative humidity. This invention is deemed to fulfill this need in an effective and efficient manner.